BBT - Not the best predictor of ovulation
BBT or Basal Body Temperature is the body temperature at rest, and lowest temperature of the body.
Why is BBT tracked to identify ovulation?
The menstrual cycle has 2 phases. These phases are a follicular phase and a luteal phase. In a 28 day cycle, the follicular phase is the first 14 days and the luteal phase is the last 14 days. The luteal phase begins after ovulation. BBT rises about 0.5 degree Fahrenheit in the luteal phase, which signifies that ovulation has occurred, and the end of the fertile period. Although the rise in BBT occurs too late to be helpful for timing intercourse, it can be useful for confirming ovulation, and predicting when menstruation will start because menstruation usually begins ~14 days after ovulation (1-2).
There is some evidence showing that a BBT dip can occur a day to a few days before ovulation. However, this may only occur in about 15% of hormonally normal cycles (3).
What do you need?
Use of a special thermometer that can distinguish readings to the one-tenth degree is needed because tracking BBT to identify ovulation requires detecting an approximate rise in 0.5 degree Fahrenheit.
How to do it?
Upon waking, and before getting out of bed, talking, drinking/eating, or going to the bathroom, take your temperature. Chart your temperature readings in a mobile app or by hand using a chart (example chart 1; example chart 2). When a rise in 0.5 degree Fahrenheit is detected, ovulation has occurred, and you can expect to begin menstruation about 2 weeks later.
BBT can be useful in conjunction with other tools, especially cervical mucus, when trying to get pregnant. Stay tuned for future posts on other fertility monitoring tools.
Jeanna Ryan MPAS, PA-C, MS, RDN, MSCIS
Physician Assistant, Registered Dietitian Nutritionist
Mother of 3 Daughters
Co-founder of PreOv
1. Jennings V. Fertility awareness-based methods of pregnancy prevention. Schreiber CA, Eckler K, ed. UpToDate. Waltham, MA: UpToDate Inc. https://www.uptodate.com
2. Welt CK. Evaluation of the menstrual cycle and timing of ovulation. Barbieri RL, Crowley WF, Martin KA, ed. UpToDate. Waltham, MA: UpToDate Inc. https://www.uptodate.com
3. Hilgers TW, Bailey AJ. Natural Family Planning. II. Basal Body Temperature and Estimated Time of Ovulation. March 1980. Obstetrics & Gynecology. 55(3): 333-339.